50 years and counting

Cosmic Ray Observatory longest running experiment in McMurdo

The U.S. Antarctic Program’s longest running experiment, which helps keeps an eye on the activity of the solar system’s hottest object, will soon be working in the cold.

The McMurdo Station Cosmic Ray Observatory first started counting the charged subatomic particles that bombard the Earth in 1960. Fifty years later, the array of neutron detectors operates much as it did then.

“What we’re really doing is measuring the intensity of the cosmic rays hitting the atmosphere, more or less directly above the instrument,” explained Paul Evenson, co-principal investigator on the CosRay experiment from the Bartol Research Institute at the University of Delaware, during a visit to the instrument this past austral summer.

The Bartol Research Institute, under well-known physicist Martin A. Pomerantz, installed the CosRay experiment at McMurdo Station. He had overseen the installation of a similar observatory three years earlier at Thule, Greenland.

In 1960, en route to McMurdo to visit the instrument for the first time, Pomerantz learned that both observatories had recorded a major solar flare event from the sun. It was the first time a solar cosmic ray event had been observed at both ends of the Earth.

“The subsequent story is just wonderful. A lot of things happened because we had that station,” Pomerantz later said during an interview in May 2000 as part of the Polar Oral History Project conducted by the American Polar Society and the Byrd Polar Archival Program of The Ohio State University. Pomerantz passed away in 2008.

However, the CosRay experiment lives on, even though the questions posed by today’s researchers have evolved.

“We’re finding new ways to use the data. We’re finding new instruments to use the data with,” Evenson said.

Counting on a prediction

So what exactly are cosmic rays and why are scientists interested in them?

First, cosmic ray is a bit of a misnomer — a holdover term from the early 20th century. Cosmic rays are energetically charged subatomic particles, originating from processes on the sun or from even more mysterious phenomena, such as a supernova explosion.

These cosmic rays speed toward the Earth at nearly the speed of light. The particles that hit the Earth, called primary cosmic rays, are destroyed when they hit the atmosphere, producing a cascade of secondary subatomic particles, including the neutrons that are detected by the CosRay Observatory.

The McMurdo observatory is one of a dozen that make up an international monitoring network called Spaceship Earth, which also includes a neutron station at the South Pole, which was first installed in 1964 (and has since been relocated twice around the U.S. research station there).

By counting the number of neutrons that arrive at these detectors, researchers can calculate how many cosmic rays arrived at a corresponding location in the Earth’s upper atmosphere.

“The main thing we’re observing is the magnetic field that comes out of the sun,” Evenson explained. “Fluctuations in the magnetic field coming out of the sun translate into the intensity of the particles we see.”

Today’s global network of neutron monitors allows the researchers to see the various asymmetries in the sun’s magnetic field. “By looking in different directions, you get a much more complete picture of the magnetic fluctuations,” he said.

Monitoring solar activity and cycles helps scientists predict when magnetic disturbances from the sun might occur. Energetic bursts from the sun can help cause “storms” in Earth’s magnetic field that can disrupt satellites. A major magnetic storm in 2003 affected 47 satellites, including one scientific satellite costing $640 million, which was written off as a total loss.

“You can gain an idea, by studying the cosmic rays, about what might be coming,” Evenson said.

CosRay experiment adapts with the times

The neutron monitor at McMurdo has 18 cosmic ray counters. They are arranged in three units, each with six tube-shaped counters. Each unit is covered with polyethylene slabs that hold six stainless steel tubes surrounded by polyethylene sleeves. Some 50 tons of lead rings surround the tubes, which are filled with a gas called boron trifluoride. A thin wire runs through the gas down the center of the tube and connects to an amplifier.

Today, a computer records the data and sends it to Bartol, which makes the information available in real-time. But, back in the old days, that job was handled by a teletype machine that recorded and printed the data. These and other pieces of equipment — including a “bit bucket” still filled with bits of paper punched out by the teletype machine — still linger in the CosRay Observatory building, a rectangular structure that sits on the road to New Zealand’s Scott Base.

The original experiment had been located in the research town proper. But construction of a nuclear power plant on the side of Observation Hill overlooking McMurdo in the early 1960s required relocating CosRay to a new site. The experiment would have detected the neutrons from the nuclear power plant, which operated for about a decade before it was shut down due to safety concerns.

“You couldn’t have a neutron monitor measuring cosmic rays next to a nuclear reactor,” Evenson noted. Its location on the other side of Ob Hill shielded it from the neutrons emitted from the reactor.

And now the venerable CosRay Observatory is once again being asked to accommodate the march of progress. But this time it’s not about nuclear energy but energy savings.

The National Science Foundation, which manages the U.S. Antarctic Program, has worked in recent years to make its facilities on the Ice more energy efficient. It collaborated with Antarctica New Zealand on a wind turbine farm, and it is experimenting with electric vehicles in McMurdo.

In another effort to save fuel, which arrives once per year on a tanker vessel, the NSF has decided to cut power to the CosRay building. Carpenters have been working on insulating the three neutron monitor units so that the experiment can continue in the cold. That project is scheduled for completion during the 2011-12 season when the sun is again predicted to get busy after a period of extreme inactivity.

Despite its age, the CosRay Observatory is still a valuable source of data, according to Evenson, who is a collaborator on an experiment associated with the IceCube Neutrino Observatory called IceTop.

IceCube is an experiment with thousands of sensors frozen into the ice sheet under the South Pole Station looking for subatomic particles called neutrinos. IceTop uses identical instruments at the surface of the array to detect cosmic ray particles at higher energies than those counted by CosRay.

Together, the two instruments from different eras can tackle new questions, such as whether the maximum particle energy in a solar flare event increases with the total energy release on the sun. In other words, do smaller solar flares produce particles the same distribution in energy as larger events?

“We’re hoping the sun gives us some events to answer some really interesting questions,” Evenson said.

NSF-funded research in this story: John Bieber and Paul Evenson, University of Delaware, Award No. 0739620; and Paul Evenson, University of Delaware, Award Nos. 0838838 and 0838839. Invaluable historical and technical resources for this article included an “Oral interview with Dr. Martin Pomerantz taken as part of the Polar Oral History Project conducted by the American Polar Society and the Byrd Polar Archival Program of the Ohio State University” and Seth White’s “Long Winded and Barely Readable But Perhaps Mildly Entertaining Polar Photo Journal.”